The invention relates to a circuit for transforming, switching, adjusting or controlling electric power, with an electric source, a load, and at least one means for transforming, switching, adjusting or controlling the electric output at the load.
In this connection, the transforming, switching, adjusting and controlling of electric power comprises all the methods or switching arrangements, respectively, by which electric power at a power mains is passed on to a consumer to a desired extent, in a desired manner and at a desired point of time. The adjustment or control may serve to provide a certain changeable power to a consumer or to provide always the same power to a changeable consumer. Also switching of electric power is included in this context. Usually, adjustment or control of electric power is effected by changing the voltage and/or the current of the source. Transforming of electric power occurs, e.g., during rectification, wherein an alternating-current voltage is transformed into a direct current voltage.
Previously, changing of the power has been obtained, e.g., by means of series resistors which have been switched on and off by means of analogous switches. Besides the limited switching frequency, such a solution also has the drawback that change can only occur in fixed steps. Moreover, series resistors involve very high losses in the form of heat which must be conducted away by cooling means. Power semiconductors used instead of analogous switches did bring certain improvements in the switching method, yet they did not reduce the losses.
In the absence of resistors, transforming, switching, adjusting or controlling is effected by means of electronic valves which change the power via the voltage or the current, e.g., by modulating the pulse widths. The term "electronic valves" refers to controlled and uncontrolled semiconductor construction elements. The controlled semiconductor construction elements are controlled either by the electric current supplied to an electrode or through electric potential applied to such an electrode. They include, e.g., bipolar and unipolar transistors, thyristors or comparable electronic construction elements. Diodes in which the current is a function of the voltage belong, e.g., to the uncontrolled electronic valves. In the connected-through state, the electronic valves have forward power losses, since in the conducting state always a certain voltage drop occurs at the valves. Although these losses are lower than those in series resistors, yet particularly in case of very high currents or very high voltages they may still be quite substantial. In such instances, these electronic valves must be dimensioned accordingly, and usually also equipped with corresponding cooling elements so that the losses which have been converted into heat can be conducted away.